Function TemporalSimplify

src/backend/utils/adt/datetime.c:4558–4581

* Common code for temporal prosupport functions: simplify, if possible, * a call to a temporal type's length-coercion function. * * Types time, timetz, timestamp and timestamptz each have a range of allowed * precisions. An unspecified precision is rigorously equivalent to the * highest specifiable precision. We can replace the function call with a * no-op RelabelType if it is coercing to

 * we can't get there from a cast: our typmodin will have caught it already.
 */
Node *
TemporalSimplify(int32 max_precis, Node *node)
{
	FuncExpr   *expr = castNode(FuncExpr, node);
	Node	   *ret = NULL;
	Node	   *typmod;

	Assert(list_length(expr->args) >= 2);

	typmod = (Node *) lsecond(expr->args);

	if (IsA(typmod, Const) && !((Const *) typmod)->constisnull)
	{
		Node	   *source = (Node *) linitial(expr->args);
		int32		old_precis = exprTypmod(source);
		int32		new_precis = DatumGetInt32(((Const *) typmod)->constvalue);

		if (new_precis < 0 || new_precis == max_precis ||
			(old_precis >= 0 && new_precis >= old_precis))
			ret = relabel_to_typmod(source, new_precis);
	}

	return ret;
}

/*
 * This function gets called during timezone config file load or reload